Cross section using Camera Clipping Distance results in very dark model interior
Make sure light can reach the interior. Use boolean modifier(s) to effectively cut away portions of your model, thus giving a true cross-section.

Example:


Create a new mesh (e.g. cube). 
Add a boolean modifier to object which you want to cut away. Set
mode "Difference". Don't hit apply.
Make the new mesh (1.) the target for the modifier. 
Move the mesh into desired position. Where the mesh overlaps the
object will be cut away.